An active galactic nucleus is a small region, between about 1 and 100 light years in diameter, at the center of a galaxy that emits prodigious amounts of radiation over some or all of the electromagnetic spectrum, from the shortest gamma rays to the longest radio waves. Active galactic nuclei are among the most energetic phenomena in the universe, beating out even supernovae in terms of luminosity.
Active galactic nuclei are powered by matter accreting onto supermassive black holes, with a mass between 106 and 1010 times that of the Sun. These black holes generally have a size about a light year in diameter. The infalling matter forms into an accretion disc, with gas particles moving and colliding with each other at velocities approaching the speed of light. This releases huge amounts of energy, often in the form of relativistic jets projecting perpendicular to the accretion disc. Depending on whether or not these jets point towards the Earth, an active galactic nucleus might have either a very great luminosity or barely luminosity at all.
Active galactic nuclei are by far most common in older, and thus more distant galaxies. Quasars, a subset of active galactic nuclei, are among the most distant observed objects in the universe, with observed distances of 13 billion light years or greater. These quasars serve as windows into the very early universe. Although it is thought that all massive galaxies have central supermassive black holes which could theoretically power active galactic nuclei, in practice it seems like only older galaxies harbor them. This is likely because the matter in the central regions of galaxies had not yet fallen into the hole and was therefore in greater supply. The most powerful active galactic nuclei consume about a solar mass worth of material every year.
Active galactic nuclei are broadly divided into radio-loud and radio-quiet AGNs. Radio-quiet active galactic nuclei tend to lack relativistic jets, releasing heat and light through accreting matter but without the projections. Radio-loud active galactic nuclei possess relativistic jets, and in the most luminous objects, blazars, these jets are located very close to the line of sight.
